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Clile C. Allen · Public domain (expired patent)

أنشأه

Emma

22. سبتمبر 2026

Varifocal Projection Objective

A lantern-slide projector threw one fixed picture size at a given distance; to fill a different screen, the lantern had to be carried closer or further. Clile C. Allen of Chicago put three lenses in one tube, fixed the first and last, and made the middle one slide on a spiral track as the tube was turned - changing the focal length, and so the picture size, from the same spot. A rack and pinion moved the whole tube to refocus. **US 696,788**, "Optical objective", Allen, filed 25. February 1901, granted 1. April 1902: "three lens elements, the first and third of which are permanently fixed … while the second is movable and capable of being placed in any intermediate position". It is one of the earliest variable-focal-length objectives - the idea behind every zoom lens. **You build it on a bench as a projector**, and measure picture size against the middle lens's position.

متوسط

2 hours

التعليمات

Draw a slide

Draw a 24 mm square grid with a marker on transparency film. Tape it over the torch lens: this is the lantern.

المواد لهذه الخطوة:

شفافيات طباعة نفث الحبر1 ورقة

شريط لاصق ورقي1 لفة

الأدوات المطلوبة:

قلم تعليم ثابت

كشّاف LED للفحص

مسطرة فولاذية

Mount three lenses

Wall side: convex ~100 mm. Slide side: convex ~50 mm, 80 mm from the first. Between them: concave ~-50 mm on a holder that slides. Aim the bench at a wall 2 m away with the lit slide behind the 50 mm lens.

المواد لهذه الخطوة:

طقم عدسات محدّبة1 طقم

عدسة مقعّرة1 قطعة

الأدوات المطلوبة:

منضدة بصرية وسكة

مجموعة حوامل عدسات

Slide, refocus, measure

Set the middle lens 10 mm from the wall-side lens. Move the slide (or the whole lens group) until the grid is sharp on the wall; measure its width. Repeat at 30 and 60 mm.

الأدوات المطلوبة:

شريط قياس

Focal length vs position

جارٍ تحميل دفتر Jupyter…

History & Context

**What the patent specifies:** a three-element objective with fixed outer elements and a movable middle element carried by a spiral track, plus a rack and pinion to move the whole tube. **Modern build (derived):** stock lenses on an optical bench. Allen's lens must be refocused after each change - a varifocal. A true zoom keeps focus while zooming by moving two groups together, which came later with cam-driven designs. Its limits: the picture dims as it grows, and aberrations change across the range.